The cytodifferentiation of spores in Dictyostelium discoideum includes the synthesis of a mucopolysaccharide composed of galactose, N-acetyl-galactosamine, and galacturonic acid which eventually makes up the outer spore coat. The mucopolysaccharide is probably synthesized within a specialized organelle called the pre-spore vacuole. Three enzymes involved in this biosynthetic pathway have been identified: UDPG-pyrophosphorylase, UDP-gal-4-epimerase and UDP-gal polysaccharide transferase. The evidence suggests that the latter two accumulate in the vacuolar lumen during development and are released when it evaginates at the cell's surface. Thus, the pre-spore vacuole is a complex organelle with considerable metabolic capabilities which can serve as a useful system for the study of the regulation of cytodifferentiation in this organism and may have broad general significance for the ontogeny of similar vesicles in other cell types including mammalian secretory vesicles. We propose to study the composition, origin, and ontogeny of the vacuoles and their biosynthetic capacities.